Electron Beam Transparent Boron Doped Diamond Electrodes for Combined Electrochemistry—Transmission Electron Microscopy

The majority of carbon based transmission electron microscopy (TEM) platforms (grids) have a significant sp2 carbon component. Here, we report a top down fabrication technique for producing freestanding, robust, electron beam transparent and conductive sp3 carbon substrates from boron doped diamond (BDD) using an ion milling/polishing process. X-ray photoelectron spectroscopy and electrochemical measurements reveal the sp3 carbon character and advantageous electrochemical properties of a BDD electrode are retained during the milling process. TEM diffraction studies show a dominant (110) crystallographic orientation. Compared with conventional carbon TEM films on metal supports, the BDD-TEM electrodes offer superior thermal, mechanical and electrochemical stability properties. For the latter, no carbon loss is observed over a wide electrochemical potential range (up to 1.80 V vs RHE) under prolonged testing times (5 h) in acid (comparable with accelerated stress testing protocols). This result also highlights the use of BDD as a corrosion free electrocatalyst TEM support for fundamental studies, and in practical energy conversion applications. High magnification TEM imaging demonstrates resolution of isolated, single atoms on the BDD-TEM electrode during electrodeposition, due to the low background electron scattering of the BDD surface. Given the high thermal conductivity and stability of the BDD-TEM electrodes, in situ monitoring of thermally induced morphological changes is also possible, shown here for the thermally induced crystallization of amorphous electrodeposited manganese oxide to the electrochemically active γ-phase.


ESI 1: Troubleshooting guide
Some common issues experienced when fabricating BDD-TEM electrodes are given in Table   S1, with suggestions on how to avoid/circumvent these.  Figure S1: Photograph of a resulting BDD-TEM electrode (3 mm in diameter) being handled with tweezers. The central hole can also be seen.

ESI 2: Uncompensated resistance measurements of BDD-TEM electrodes
In a potential range where no faradaic reactions occur, the electrode can be treated as ideally polarizable. Under these conditions, the current flowing, i, in response to a potential pulse, ΔE is described by eq. S1: 1,2 where Ru is the uncompensated resistance (Ω), t is the total time (s), and C is the capacitance (F). Ru measurements were made using ΔE of 0.1 V (0.20 V to 0.30 V vs Ag/AgCl) for 5 ms in 0.1 M KNO3. Five separate pulses were recorded for each electrode contact, and the i-t response recorded, Figure S2. Each pulse was fitted according to eq. S2 following the Levenberg Marquardit iteration algorithm: where b is equal to -1/RuC. Ru can then be calculated via eq. S3, where a is the pre-exponential term from the fitted function (eq. S2):      Table S3.
Binding energies have been considered relative to the assigned sp 3 C-C/C-H peak for each electrode. The C 1s spectra of a commercial TEM grid, an amorphous C film floated onto an Au support mesh (Agar Scientific), was acquired following the same experimental procedure as the BDD control sample. Figure S5 shows the fitted C1s spectra, and Table S3 shows the relative contributions of each peak expressed as percentages of the fitted envelope. It is clear from the C 1s fitting that this commercial grid has a high sp 2 carbon content (as expected), in this case 62%, and a much lower sp 3   π -π * 290.7 1.9 S10

ESI 5: Contact angle measurements
Contact angle measurements were recorded (in triplicate) to compare the hydrophobicity and wetting of a BDD-TEM electrode vs a commercial amorphous carbon coated TEM substrate, C/Au TEM (Fig. S6). The average contact angles measured were 62.2 ± 0.5° and 83.6 ± 1.1° for the BDD and (amorphous) carbon substrates, respectively.

ESI 6: Electrochemical Characterization
The solvent windows for both samples in 0.1 M KNO3 at 0.1 V s -1 were wide and featureless ( Fig. S7a), with values of 3.18 and 3.54 V (for a given geometric current density of ±0.4 mA cm -2 ) for the BDD-TEM and BDD control electrodes, respectively. To calculate the electrochemical capacitance, C, the voltage window was decreased to 0 V ± 0.1 V (Fig. S7b) and equation S4 was used: where iav is the average current magnitude at 0 V from the forward and reverse sweep, ν is the scan rate (0.1 V s −1 ) and A is the geometric electrode area. Capacitance values of 5.3 and 4.3 µF cm -2 for the ion milled/polished and mechanically polished electrodes, respectively, were measured. For mechanically polished CVD-grown BDD a C of ≤10 μF cm −2 is typical (acquired using digital staircase CV). 47 The one-electron reduction of Ru(NH3)6 3+ was also studied by CV (Fig. S7c). For the mechanically polished electrode, a peak-to-peak separation, ΔEp, of 68 mV was measured, compared to 70 mV, for the ion milled/polished electrode. These responses are close to reversible.   (Table S5) (Fig. S11). For the BDD-TEM grids no visual changes were observed (Fig.   S11a), and the same grid could be used for all the heating studies. A new C/Cu commercial TEM grid was used for each temperature as the grid was visibly damaged after heating (Fig.   S11b), including holes in the carbon film. After heating for 4 hours at 400°C, most of the squares in the TEM grid are devoid of C film (Fig. S11biii). Figure S11. Photographs of (a) a BDD-TEM electrode (i) before heating in air and (ii) after heating in air to 200°C and then 400°C for 4 hours at each temperature, and b) a C/Cu commercial TEM grid (i) before heating in air and (ii-iii) after heating in air for 4 hours to ii) 200°C, and iii) 400°C. A fresh C/Cu grid was used for each annealing temperature due to induced damage in each heating experiment. The same BDD-TEM electrode was used throughout with annealing temperature increasing from lowest to highest. Note all TEM grids are 3 mm in diameter.